474

IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 21, NO. 7, APRIL 1, 2009

One-End-Access High-Speed Distributed Strain
Measurement with 13-mm Spatial Resolution
Based on Brillouin Optical Correlation-Domain
Reflectometry
Yosuke Mizuno, Zuyuan He, Member, IEEE, and Kazuo Hotate, Fellow, IEEE

Abstract—We demonstrate a one-end-access high-speed distributed strain sensing with high spatial resolution in optical fibers
based on Brillouin optical correlation-domain reflectometry. In
this work, Brillouin frequency shifts (BFSs) of a 3-cm fiber section
are measured with 13-mm spatial resolution and 50-Hz samplingrate. To our knowledge, this spatial resolution is the best result
ever reported in Brillouin scattering-based reflectometers, andis superior to that obtained in Brillouin optical time-domain
analyzers, though the signal-to-noise ratio is deteriorated. Data
processing to find the peak in the Brillouin spectrum is newly
introduced to show directly the BFS with a sampling rate of 19 Hz,
and the measurement of vibrated strains at frequencies of up to
4 Hz is presented under 22-cm resolution.
Index Terms—Brillouin scattering, correlation, distributed measurement, modulation, nonlinear optics, real-time systems, reflectometry, signal resolution.

I. INTRODUCTIONPTICAL fiber sensors based on Brillouin scattering haveattracted much attention due to the capability to measure
the distribution of strain and/or temperature in structures andmaterials [1]–[13]. They are classified into two types: “reflectometers” [5]–[8], in which a light beam is injected into one
end of the fiber under test (FUT), and “analyzers” [9]–[13],in which two light beams are injected into both ends of the
FUT. A Brillouin optical correlation-domain reflectometer(BOCDR), which has been recently proposed by the authors[8], is one of the former sensors. Though analyzers can obtain
relatively high signal-to-noise (S/N) ratio and short acquisition
time due to utilizing the stimulated Brillouin scattering, theycannot work completely when the FUT has even one breakage
point. For some practical applications, one-end access reflectometers are more favorable than analyzers. In conventional
pulse-based Brillouin optical time-domain reflectometers

O

Manuscript received September 17, 2008; revised December 11, 2008. First
published February 03, 2009; current version published March 18, 2009. This
work was supported by the “Grant-in-Aid for Creative Scientific Research” andby the “Global Center of Excellence Program” from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. The work of
Y. Mizuno was supported by the Japan Society for the Promotion of Science(JSPS) Research Fellowships for Young Scientists.
The authors are with the Department of Electronic Engineering, Universityof Tokyo, Tokyo 113-8656, Japan (e-mail: ymizuno@sagnac.t.u-tokyo.ac.jp;
ka@sagnac.t.u-tokyo.ac.jp; hotate@sagnac.t.u-tokyo.ac.jp).
Color versions of one or more of the figures in this letter are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/LPT.2009.2013643(BOTDRs) [5]–[7], the measurement range is as long as several
tens of kilometers, but the best spatial resolution is typically
1 m [6], though some progress has been made toward further
enhancement [7]. This is because there is a trade-off between
the spatial resolution and the Brillouin frequency shift (BFS)
resolution [6]. Moreover, the signal reflected from one opticalpulse is so small that a large number of pulses have to be
integrated for observable read-out. Therefore, the measurement
time of BOTDR becomes as long as several minutes, which is
insufficient for such applications as real-time health monitoringof various structures.
Recently, we have proposed a BOCDR [8] to measure thestrain and/or temperature distribution along an FUT by controlling the interference of continuous lightwaves. The BOCDR
system is based on spontaneous Brillouin scattering, and shows
unique features such as random access to measuring positions,high spatial resolution, and high-speed measurement, though it
has a trade-off relation between the spatial resolution and themeasurement range. In our previous preliminary experiment,
40-cm spatial resolution was obtained, where the sampling rate
was 50 Hz corresponding to the sweep rate of an electrical spectrum analyzer (ESA) used to detect the signal [8].
In this letter, we challenge the limitation of the spatial resolution in BOCDR, and demonstrate a distributed strain measurement on a fiber section as small as 3 cm with 13-mm resolution.
To the best of our knowledge, this is the highest spatial resolution that has ever been reported in spontaneous Brillouinscattering-based strain-sensing reflectometers, and is even superior to that obtained in stimulated Brillouin scattering-based
Brillouin optical time-domain analyzers (BOTDAs) [9] (thehighest resolution reported so far was 20 mm by dark-pulse
BOTDA [10]), though the S/N ratio is deteriorated. In addition,
to confirm the dynamic measurement capability of BOCDR,
we also introduce the data processing to show directly the BFS,
and demonstrate the detection of vibrated strains at frequenciesof up to 4 Hz under 22-cm resolution.
II. PRINCIPLE
When a light beam is injected into an optical fiber, backscattered light (Stokes light) is generated through the interaction
with acoustic phonons. This phenomenon is called spontaneous
Brillouin scattering [14]. The spectrum of the Stokes light,
also known as the Brillouin gain spectrum (BGS), takes theshape of Lorentzian function. The frequency at which the peak
power is obtained in the BGS is about 11-GHz down-shifted1041-1135/$25.00 © 2009 IEEE
Authorized licensed use limited to: IEEE Xplore. Downloaded on April 22, 2009 at 23:11 from IEEE Xplore. Restrictions apply.

MIZUNO et al.: ONE-END-ACCESS HIGH-SPEED DISTRIBUTED STRAIN MEASUREMENT WITH 13-mm SPATIAL RESOLUTION

475

Fig. 2. (a) FUT prepared by fixing an SMF on the translation stage with epoxy
glue. (b) Schematic of the structure of the FUT.

Fig. 1. Experimental setup of the BOCDR system. AC: alternating current;
DAQ: data acquisition; DC: direct current; DFB-LD: distributed-feedback LD;
GPIB: general-purpose interface bus; PC: personal computer; PD: photodiode.

from the incident light frequency when the incident light wavelength is 1.55 m. This amount of frequency shift is called, and it varies in proportion to the applied tensile
the BFS
MHz/%) or temperature change
strain (0.05 MHz/
(1 MHz/K) [1].
The basic principle of the BOCDR system is to select thestate of spontaneous Brillouin scattering over one specific section from the FUT by applying the same frequency modulation
to the reference and the Stokes light, producing periodical correlation peaks along the FUT [8], [13]. The spatial resolution
and the measurement range (i.e., interval between correlaare given, by the same equations as those fortion peaks)
BOCDA [11], as
(1)
(2)
where
is the group velocity of light in the fiber,thetheintrinsic linewidth of BGS ( 30 MHz in fibers), andis the modulamodulation frequency of the light source;tion amplitude, which is limited to a half of the BFS, i.e., about
5.4 GHz in BOCDR [8]. According to (1) and (2), the spatial
further at the costresolution can be enhanced by increasingof the measurement range in principle. However, with the enhanced resolution, the intensity of the Brillouin-scattered signal
from the corresponding fiber section is reduced, which leads todecreased S/N ratio in the measurement, because the noise-floor
in the electrical region is fixed. Thus, there exists a limitation to
the spatial resolution in the BOCDR system.
III. EXPERIMENT AND RESULTS
The experimental setup of BOCDR is depicted in Fig. 1. A
1552-nm distributed-feedback laser diode (LD) at frequency
was used as a light source, and a sinusoidal frequency modulation was applied to generate a correlation peak within an FUT.
The output from the LD was divided into two light beams bya coupler. One of the beams was directly used as the reference
light of self-heterodyne detection, after passing a 2-km delay
fiber for controlling the order of the correlation peak, an erbiumdoped fiber amplifier (EDFA) for enhancing the heterodyne beatsignal, and an optical filter composed of fiber Bragg grating witha 3-dB bandwidth of about 10 GHz for suppressing the amplified spontaneous emission (ASE) noise. The other beam wasFig. 3. (a) Distribution of the BGS along the FUT. (b) Distribution of the
BFS along the FUT. Spatial resolution better than 3 cm was achieved with
0.02 s/point sampling rate. A resolution of one-half (15 mm) of the marked
width is obviously available.

injected into the FUT as the pump light, after being amplified
by a high-power EDFA to 28 dBm. The weak Stokes light atbackscattered from the FUT was amplified
frequency –again by an EDFA. A precisely adjusted optical filter was inserted after the EDFA in order to suppress the noise associatedwith the Rayleigh scattering from the FUT at frequency [8].
The optical beat signal of the reference light and the Stokes light
was converted by balanced photodiodes to an electrical signal.
After 15-dB amplification from an electrical preamplifier, thesignal was monitored by the ESA. The measurement data weretransferred to a personal computer.
was 13.4624–13.4672 MHz,The modulation frequencyof 7.6 m acwhich corresponds to a measurement range
cording to (2). The amplitude of the frequency modulation
was 5.4 GHz, and the nominal spatial resolution
was calculated to be about 13 mm from (1). The 128th correlation peak
was used. Fig. 2(a) and (b) shows the structure of the FUT comprising a 5-m standard single-mode fiber (SMF), in which a

Authorized licensed use limited to: IEEE Xplore. Downloaded on April 22, 2009 at 23:11 from IEEE Xplore. Restrictions apply.

476

IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 21, NO. 7, APRIL 1, 2009

results are shown in Fig. 4, where the strain oscillations areclearly observed. The amplitude discrepancy seems to havecome from the mitigation effect by the fiber jacket. The accuracy of the measurement is limited by the electrical noise of the
ESA as well as the instability of polarization state.
IV. CONCLUSION
We demonstrated a one-end-access distributed strain measurement with 13-mm spatial resolution based on Brillouin optical correlation-domain reflectometry. The BFSs of a 3-cm fiber
section were measured, though S/N ratio was deteriorated. Sucha high resolution and a high sampling rate have not yet been realized by other Brillouin scattering-based reflectometers as wellas two-end-access time-domain analyzers. Data processing tofind the peak in the BGS was newly introduced to show directly
the BFS with a sampling rate of 19 Hz, and a detection of vibrated strains at frequencies of up to 4 Hz was presented under
the resolution of 22 cm.
Although BOCDR has an advantage of one-end accessibility,
its performance has not come up to that of BOCDA [11], [12]yet. On this point, we think further improvement is needed.
Fig. 4. Results of dynamic strain measurement with sinusoidal driving voltages
applied to the PZT at (a) 1 Hz, (b) 2 Hz, and (c) 4 Hz, respectively. Note thatthe sampling rate is 19 Hz.

REFERENCES

0.1% strain was applied to a 3-cm section fixed on a translation
stage with epoxy glue. One end of the FUT was spliced to a circulator, and the other end was microbent to suppress the Fresnel
reflection. The overall sampling rate of the measurement for asingle position was 50 Hz, which is quite high compared withthe time-domain techniques.
Fig. 3(a) shows the measurement result of the distribution of
the BGS along the FUT. The BGS at the strain-applied section
is recognized. Fig. 3(b) shows the distribution of the BFS. We
can see that a spatial resolution better than 3 cm is achieved, anda resolution of one-half (15 mm) of the marked width is obviously available. It is in good agreement with the calculated valueof 13 mm. The change of the BFS was about 50 MHz, which
agrees well with the applied strain of 0.1%. The accuracy of themeasurement at a single position was about 5 MHz, which corresponds to the strain of 0.01% ( 100 ) in this experiment.
However, the S/N ratio for this case was deteriorated, so that thepeak of the BGS was mostly buried by the electrical noise floorof the ESA if the resolution was set even higher. As a method
to enhance the resolution further, new schemes such as lock-in
detection must be developed.
Next, we confirmed the real-time measurement capability of
the BOCDR by a dynamic strain sensing experiment. Whendata processing to find the peak in the Brillouin spectrum wasintroduced, the maximum sampling rate was 19 Hz. In order toand theobtain a higher S/N ratio, the modulation frequency
were set to 828.26 kHz and 5.4 GHz,
modulation amplituderespectively, which correspond to 22-cm spatial resolution
and 124.9-m measurement range. A 5-m SMF was used as an
FUT, in which dynamic strains were applied to a 28-cm section(420–448 cm) by a piezoelectric transducer (PZT) with themost achievable strain of about 450( 22.5 MHz in BFS).
Sinusoidally modulated voltages were applied to the PZT at thefrequencies of 1, 2, and 4 Hz, respectively. The measurement[1] T. Horiguchi, T. Kurashima, and M. Tateda, “A technique to measure
distributed strain in optical fibers,” IEEE Photon. Technol. Lett., vol. 2,
no. 5, pp. 352–354, May 1990.
[2] X. Bao, D. J. Webb, and D. A. Jackson, “32-km distributed temperature sensor using Brillouin loss in optical fiber,” Opt. Lett., vol. 18, pp.
1561–1563, 1993.
[3] M. Nikles, L. Thevenaz, and P. Robert, “Simple distributed fiber sensor
based on Brillouin gain spectrum analysis,” Opt. Lett., vol. 21, pp.
758–760, 1996.
[4] M. N. Alahbabi, Y. T. Cho, and T. P. Newson, “150-km-range distributed temperature sensor based on coherent detection of spontaneous
Brillouin backscatter and in-line Raman amplification,” J. Opt. Soc.
Amer. B, vol. 22, no. 6, pp. 1321–1324, Jun. 2005.
[5] T. Kurashima, T. Horiguchi, H. Izumita, S. Furukawa, and Y. Koyamada, “Brillouin optical-fiber time domain reflectometry,” IEICE
Trans. Commun., vol. E76-B, no. 4, pp. 382–390, Apr. 1993.
[6] A. Fellay, L. Thevenaz, M. Facchini, M. Nikles, and P. Robert, “Distributed sensing using stimulated Brillouin scattering: Towards ultimate resolution,” in Proc. 12th Int. Conf. Opt. Fibre Sensors OSA Tech.
Dig. Series, 1997, vol. 16, pp. 324–327.
[7] Y. Koyamada, Y. Sakairi, N. Takeuchi, and S. Adachi, “Novel technique to improve spatial resolution in Brillouin optical time-domainreflectometry,” IEEE Photon. Technol. Lett., vol. 19, no. 23, pp.
1910–1912, Dec. 1, 2007.
[8] Y. Mizuno, W. Zou, Z. He, and K. Hotate, “Proposal of Brillouin optical correlation-domain reflectometry (BOCDR),” Opt. Express, vol.
16, no. 16, pp. 12148–12153, Jul. 2008.
[9] T. Horiguchi and M. Tateda, “BOTDA-nondestructive measurementof single-mode optical fiber attenuation characteristics using Brillouininteraction: Theory,” J. Lightw. Technol., vol. 7, no. 8, pp. 1170–1176,
Aug. 1989.
[10] A. W. Brown, B. G. Colpitts, and K. Brown, “Distributed sensor basedon dark-pulse Brillouin scattering,” IEEE Photon. Technol. Lett., vol.
17, no. 7, pp. 1501–1503, Jul. 2005.
[11] K. Hotate and T. Hasegawa, “Measurement of Brillouin gain spectrum distribution along an optical fiber using a correlation-based technique—Proposal, experiment and simulation,” IEICE Trans. Electron.,
vol. E83-C, no. 3, pp. 405–412, Mar. 2000.
[12] K. Y. Song, Z. He, and K. Hotate, “Distributed strain measurementwith millimeter-order spatial resolution based on Brillouin optical correlation domain analysis,” Opt. Lett., vol. 31, pp. 2526–2528, 2006.
[13] K. Hotate and Z. He, “Synthesis of optical-coherence function and its
applications in distributed and multiplexed optical sensing,” J. Lightw.
Technol., vol. 24, no. 7, pp. 2541–2557, Jul. 2006.
[14] G. P. Agrawal, Nonlinear Fiber Optics. San Diego, CA: Academic,
1995.

Authorized licensed use limited to: IEEE Xplore. Downloaded on April 22, 2009 at 23:11 from IEEE Xplore. Restrictions apply.